Method and apparatus for detecting a defective pressure switch

ABSTRACT

The invention provides a method of detecting a defective condition on a pressure switch arrangement which at least assumes a defective condition on the said pressure switch assembly whenever a signal arrives at an input of an electronic computing unit which cannot be associated either to the opened or to the closed condition of the pressure switch. Moreover, a defective condition on the pressure switch arrangement mounted in the brake system of a vehicle is assumed, once the pressure switch signal refers to an actuated brake although the vehicle is significantly accelerated. Conversely, a defective condition is also assumed if the vehicle significantly decelerates although the pressure switch signal refers to a lack of brake pressure. Also, an appropriate pressure switch arrangement is provided which, with the pressure switch closed, generates a rectangular signal and, with the pressure switch opened, generates a high level on the input II of an electronic computing unit. With an incoming low level, an error on the pressure switch arrangement is assumed.

BACKGROUND OF THE INVENTION

The present invention relates to a method of detecting a defectivecondition on a pressure switch assembly and to an arrangement fordetecting a defective condition of a pressure switch assembly.

Pressure switches of the afore-mentioned type are mounted, for example,in automotive vehicles comprising a front axle/rear axle brake circuitdivision. In the event of a brake circuit failure on the front axle,they insure that a brake pressure reduction on the rear axle foradjusting the ideal brake force distribution in that case be preventedfrom occurring. As a rule, a pressure switch, with the brake notactuated, is closed and is opened by the brake pressure. Hence, if thepressure switch during a deceleration process, for example, with thebrake light switch actuated, remains closed, this is indicative of thatno pressure is built up in the front axle brake circuit. The brakecircuit is defective. In order that the ECE directive requiring a 30%deceleration of the vehicle in the event of a brake circuit failure, canalso be complied with by the rear axle brake circuit alone, in such anevent of a detected defective condition of the front axle brake circuit,a brake force distribution is prevented by a conventional brake pressurereducer lockable by electrical means, or by cutting off a means for theelectronic brake force distribution. Hence, a brake force control can beperformed only with the pressure switch opened because this alone willinsure that pressure is built up in the front axle. Should the pressureswitch be opened only on account of a defective condition, also in adefective front axle brake circuit a brake pressure reduction would takeplace on the rear axle so that the required deceleration of theautomotive could not be attained. For safety reasons, also otherdefective conditions of the pressure switch should be detected. It isthe object of the invention to provide a method of the afore-mentionedtype as well as an appertaining pressure switch assembly, which methodenables detection of a defective condition of the pressure switcharrangement.

SUMMARY OF THE INVENTION

A method solving the afore-described problem is described by the presentinvention. A pressure switch arrangement generating correspondingcontrollable signals is also part of the present invention.

An error is recognized in the simplest way if the signal transmitted bythe pressure switch arrangement to the controller does not fit with oneor the other operating condition of the brake.

In the event that, additionally, an acceleration of the vehicle isdetected or computed, there is another capability to check, in responseto the acceleration, the plausibility of the signal currently generatedby the pressure switch. Non-plausible is, for example, the display of anactuation of the brake although the vehicle significantly accelerates.An acceleration of the vehicle of at least approximately 0.2 g is deemeda significant acceleration.

Conversely, also a non-plausible signal is available if the pressureswitch displays lack of brake pressure although the vehiclesignificantly decelerates, especially so if actuation of the brake isdisplayed on account of an electronic brake pressure regulation or abrake light switch. A significant deceleration in this respect is adeceleration that cannot be attained by the rear axle brake circuitalone so that it will have to be assumed that a brake pressure has beenbuilt up in the front axle brake circuit contrary to the signalgenerated by the pressure circuit which applies in the event of adeceleration from about 0.6 g onward.

With a defective condition of the pressure switch detected, it isadvisable to activate an optical display, such as a pilot lamp.

Also, an emergency function of a brake slip control or of anotherelectronic brake pressure control can be released. However, also in thatcase the driver should be warned as in an actual failure of the frontaxle brake circuit no additional detection of the defective conditionwill occur.

The pressure circuit arrangement according to the present invention isable to generate three different signals, depending on the operatingcondition of the pressure switch. A pulsated signal indicates a pressurebuilt-up, a constantly high level is indicative of a non-pressurizedbrake circuit, while a constantly low level indicates a mass shortcircuit.

The invention will now be described more closely with reference to oneform of embodiment in conjunction with the drawing.

BRIEF DESCRIPTION OF THE DRAWINGS

The single depict shows a pressure switch assembly of the invention.

DETAILED DESCRIPTION OF THE DRAWINGS

The electronic computing unit 1 designated by ECU includes twoconnectors I and II for the pressure switch arrangement. Connector I isan output on which is generated a rectangular signal 2. Inserted intothe conduit branch 3 in communication with connector I is a pressureswitch 4. The pressure switch 4, in its basic position, is closed and isopened by the hydraulic pressure that has built up.

The second connector II is an input to the electronic computing unit.The conduit branch 5 in communication with the connector II extends to ajunction point 6 to which is also connected the conduit branch 3.Moreover, connected to junction point 6 is a conduit 7 inserted in whichis a so-called “pull-up resistor” 8 which is designated by referencecharacter R. The conduit 7 connects the junction point 6 to the pluspole 9 of a constant voltage source, for example, a vehicle battery.

With the pressure switch 4 closed, the rectangular signal 2 arrives atconnector II. With the pressure switch 4 opened, a high level is appliedto connector II on account of the pull-up resistor 8 in communicationwith the constant voltage source.

If, with the pressure switch opened, a mass short circuit has occurredin conduit 5, a low level will arrive at connector II which, should thiscondition prevail for a certain period of time, would be indicative of adefective pressure switch, because the rectangular signal 2 exhibits lowlevels for very short periods of time only. With a pressure switchclosed, low level would equally arrive at connector II, should a massshort circuit have occurred somewhere between connector I and connectorII. Also in that case, a defective pressure switch will be recognized.However, if in lieu of a mass short circuit a plus exists betweenconnectors I and II, for example, on account of a defective condition ofthe pull-up resistor 8, this would not result in detection of an errorof the pressure switch assembly according to the method of theinvention.

Additional plausibility checks are, therefore, provided.

If the vehicle, over a predetermined period of time, passes through anacceleration of at least 0.2 g, it will be checked whether the pressureswitch is open, i.e. whether a high level arrives at connector II.Should this be the case, the pressure switch or the pressure switcharrangement would be deemed defective because an open pressure switchrefers to an actuated brake.

Conversely, with a permanently closed pressure switch, which isindicative of that the brake is not applied, the vehicle deceleration iswatched. If in that case the vehicle is decelerated by more than 0.6 gand if this condition continues for a predetermined time without openingthe pressure switch, the pressure switch is also deemed defectivebecause such a significant vehicle deceleration can only be achieved bytwo intact brake circuits. This will especially apply, if additionalparameters are indicative of an actuation of the brake, such as actuatedbrake light switch or electronic brake pressure control in anti-lockingoperation, etc.

This error detection of the pressure switch arrangement can, of course,also be employed with such pressure switches that are closed uponactuation of the brake. In that case only the signals for the actuatedand non-actuated brake will be exchanged.

In all three checks of the pressure switch assembly, the error detectionresults in activating the pilot lamp, which is intended to indicate tothe driver that, for example, the function of an electronic brake forcedistribution is limited. The electronic brake force distribution must,therefore, be re-switched to a less sensitive control because in theevent of an additional failure of the front axle brake circuit thestandards nevertheless have to be fulfilled, according to which anadequate deceleration is to be achieved with the rear axle brake circuitalone. An anti-locking system can continue to work with no changeseffected.

What is claimed is:
 1. A method of detecting a defective condition of apressure switch assembly arranged within a pressure fluid-actuated brakesystem of a vehicle which comprises: the step of transmitting a firstsignal to an electronic computing unit if no brake pressure has builtup; the step of transmitting a second signal differing from the firstsignal to the electronic computing unit if a brake pressure has builtup; the first and second signals to the electronic computer unit by asingle connecting conduit; and wherein the electronic computing unitindicates a defective condition for a pressure switch for a frontaxle/rear axle circuit division if a signal not corresponding to eitherthe first or second signal is received.
 2. A method according to claim1, further comprising the step of activating an optical display if adefective condition is indicated.
 3. A method according to claim 1,wherein if the second signal is transmitted to the electronic computingunit, the method further includes monitoring the second signal for apredetermined period of time while the vehicle significantly acceleratesto indicate a defective condition.
 4. A method according to claim 1,wherein if the first signal is transmitted to the electronic computingunit while the vehicle is so significantly decelerated that such adeceleration can be achieved only with two intact brake cicuits, themethod further includes monitoring the first signal to indicate adefective condition.
 5. The method of claim 1, wherein said first signalis a pulsated square signal to indicate a non-pressurized brake ciruit.6. The method of claim 1, wherein said second signal is constantly highlevel signal to indicate a pressure build up.
 7. An arrangement fordetecting a defective condition of a pressure switch assembly located ina pressure fluid-actuated brake system and transmitting a first signalto an electronic computing system if no brake pressure has built up, anda second signal differing therefrom if brake pressure has built up, thearrangement comprising: an electronic computing unit; an outputconnecting conduit connected to an output port of the electroniccomputing unit, with a pulsated voltage being applied to the outputport; an input connecting conduit connected to an input port of theelectronic computing unit; the output connecting conduit and the inputconnecting conduit being connected to each other at a junction point; apressure switch provided in the output connecting conduit; and a pull-upresistor having a first end and a second end, the first end connected tothe junction point and the second end connected to a constant voltagesource, with a constant voltage being applied to the input port throughthe resistor.